Tolerate null repo URLs

[HebaruSan]

Problem

A Discord user reported an exception at startup:

5393 [1] ERROR CKAN.GUI.Main (null) - Object reference not set to an instance of an object.
System.NullReferenceException: Object reference not set to an instance of an object.
   at CKAN.RepositoryDataManager.<>c__DisplayClass7_0.<Update>b__1(Repository r)
   at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at CKAN.RepositoryDataManager.Update(Repository[] repos, IGame game, Boolean skipETags, NetAsyncDownloader downloader, IUser user)
   at CKAN.GUI.Main.UpdateRepo(Object sender, DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)

Cause

A code audit suggests that this exception could be thrown if Repository.url is null, which is not supposed to be possible but could happen if registry.json has the repo URL set to the empty string. (The deserializer throws an exception instead if it's null in the file, so we can rule that out.)

However, when I tried this, I got many other exceptions in other earlier spots in the code that access this property. At time of writing we have asked follow-up questions for further investigation but haven't heard back yet. The same user said they saw an earlier error message relating to GUIConfig.xml (we don't know which one), so it's possible they're experiencing some disk corruption.

Changes

• Now various places that access Repository.url are updated to handle null values better.
• If you have no repositories at all when you attempt a refresh, a default repo called default was being added. Now it's changed to <Game shortname>-<localized equivalent of "default">, consistent with the repo we initially add when you add the instance.